1. Field of the Invention
The present invention relates to methods and compositions for increasing stress tolerance in plants. More specifically, the present invention provides methods and compositions for improving drought resistance, salt tolerance, and resistance to freezing.
2. Background of the Invention
Plants respond to environmental challenges in part by altering their gene expression profile that ultimately leads to various adaptive responses at cellular and whole plant levels (Bray, 1993; Thomashow, 1999; Hasegawa et al., 2000; Zhu et al., 1997; Ingram and Bartel, 1996). One important regulator of plant responses to abiotic stress environments is the phytohormone abscisic acid (ABA). ABA is involved in plant responses to abiotic stress such as low temperature, drought, and salinity, as well as the regulation of plant growth and development including embryogenesis, seed dormancy, shoot and root growth and leaf transpiration (Koornneef et al., 1998; McCourt, 1999; Leung and Giraudat, 1998; Rock, 2000). Evidence for a role of ABA in stress-responsive gene regulation in plants has been two-fold. Firstly, under cold, drought, or salt stress conditions, plants accumulate increased amount of ABA, with drought stress having the most prominent effect on ABA accumulation. Secondly, the expression of many stress-responsive genes is induced by exogenous ABA, and their stress-inducibility is decreased in mutant plants defective in ABA biosynthesis or responsiveness.
Genetic analysis based on the inhibitory effect of ABA on seed germination has yielded mutants with reduced ABA biosynthesis or altered ABA responsiveness (Koornneef et al., 1998; McCourt, 1999; Leung and Giraudat, 1998; Rock, 2000). The former group of mutants in Arabidopsis includes aba1, aba2, and aba3. The ABA1 gene encodes a zeaxanthin epoxidase that functions in an early step of ABA biosynthesis by converting zeaxanthin to violaxathin. Molecular cloning of ABA2 or ABA3 has not been reported thus far. Common phenotypes of these aba mutants include loss in seed dormancy, germination resistance to NaCl stress, and withering when transferred from high humidity to low humidity conditions. The utilization of ABA deficient mutants along with ABA response mutants in stress gene regulation studies led to the notion that stress-responsive gene expression in plants is mediated by both ABA-dependent and ABA-independent pathways (Shinozaki and Yamaguchi-Shinozaki, 1997; Leung and Giraudat, 1998; Rock, 2000; Thomashow, 1999). Although the molecular mechanisms underlying the differences between ABA-dependent and ABA-independent gene regulation is unclear, analysis of the promoters of stress-responsive genes and the isolation of transcription factors that activate these genes support that there are distinct regulatory mechanisms for the different pathways. The ABRE (ABA-responsive element) complex in these promoters mediates gene induction by ABA (Guiltinan et al., 1990; Yamaguchi-Shinazaki and Shinozaki, 1994; Shen and Ho, 1995; Vasil et al., 1995), whereas the DRE/CRT (dehydration-responsive element) mediates cold and osmotic stress responsiveness independently of ABA (Yamaguchi-Shinozaki and Shinozaki, 1994; Stockinger et al., 1997). Despite these differences in transcriptional activation, genetic analysis has indicated that the ABA-dependent and ABA-independent pathways have extensive interactions or crosstalk in controlling gene expression under abiotic stresses (Ishitani et al., 1997; Xiong et al., 1999a).
As is well-appreciated in the field, there remains a need for methods of improving the resistance of plants to drought.